A printed circuit (PC) board generally comprises numerous electrical components mounted to a substrate. Two common types of components are the lead frame component and the flip chip, both of which can be mounted by different techniques to the substrate. As the name suggests, the lead frame component is connected to the substrate via individual wires extending from the substrate and connecting to the leads of the lead frame component. In contrast, the flip chip is connected to the substrate without the use of any wire connectors. For smaller and smaller electrical components, the flip chip is preferred because it is more easily attached to the substrate than the lead frame component using wire connections.
A flip chip generally has an array of solder balls or bumps affixed to the underside or image side of the flip chip. The substrate has complementary interconnect or solder pads registered to align with the solder balls. To attach the flip chip to the substrate, a flux is applied to the substrate. The flip chip is then placed onto the substrate such that the solder balls rest upon the solder pads. With the flip chip resting upon the substrate, the entire PC board is heated until the solder balls reflow to mechanically and electrically connect the flip chip to the substrate. The PC board is then removed from the heat and an underfill material is inserted in the gap between the flip chip and the substrate. After the underfill material is deposited into the gap, the entire PC board is again reheated in order to cure, i.e., fix, the underfill material.
Typically, the underfill material is highly flowable and is deposited onto the substrate next to the flip chip and allowed to wick, via capillary action, into the gap between the flip chip and the substrate. To facilitate this underfill process, the underfill material is dispensed through a needle positioned very close to the substrate surface and directly next to the flip chip itself. The height of the dispensing needle is critical to the proper placement of the underfill material between the flip chip and the substrate. To be most effective, the underfill material dispensed from the dispensing needle must make contact with the substrate while it continuously discharges from the dispensing needle. Consequently, the dispense needle tip must be very close to the substrate surface. To achieve this minimum spacing between the dispense needle tip and the substrate, a height sensor must be employed. Typically, a height sensor is placed directly onto the substrate as the underfill material is dispensed from the dispensing needle. The height sensor contacting the substrate surface can cause deflection of the substrate. If severe enough, the deflection may cause height sensing errors such that the needle is incorrectly positioned relative to the substrate. For example, the deflection may cause the needle to be positioned too low and may even contact the substrate.
To ensure that wicking draws the flowable underfill into the gap, the dispense needle must be positioned extremely close to the edge of the chip so that the dispensed material contacts the edge of the flip chip. Consequently, if the needle is incorrectly positioned because of vision errors with the positioning system or if the needle is bent because of contact with the substrate, the needle may contact the edge of the chip. This contact may damage the edge of the chip, especially near the corner of the flip chip.
To alleviate some of the problems associated with using flowable underfills after the flip chip is already attached to the substrate, no-flow flux and underfill mixtures may be employed. However, to use the no-flow flux and underfill, a different application process must be used. For instance, prior to placing the flip chip onto the substrate, a dispense needle dispenses a predetermined amount of no-flow flux and underfill material onto the designated array of solder pads. Once the no-flow flux and underfill is dispensed onto the substrate, the flip chip is pushed down into the no-flow flux and underfill material until the solder balls contact the solder pads. The PC board is then heated until the solder balls reflow to mechanically and electrically connect with the solder pads. During the heating step, the flux component assists in soldering the flip chip to the substrate. In addition, the underfill material is cured and no subsequent reflowing is required. By using this mixture of no-flow flux and underfill, the additional heating step normally required to cure the underfill material is eliminated. Consequently, the manufacturing process of the printed circuit boards is much faster, less complicated and more efficient.
However, the use of the dispense needle to apply the no-flow flux and underfill material to the substrate prior to applying the flip chip has disadvantages. As explained above, any time underfill material is dispensed from a dispense needle, the dispense needle must remain relatively close to the substrate surface. This requires that a height sensor be used to maintain the proper distance between the tip of the dispense needle and the substrate surface. As discussed above, the contact force of the height sensor may damage the substrate surface. In addition, the topography of the no-flow flux and underfill material relative to the surface of the substrate cannot be controlled or tailored. That is, the dispense needle must remain at a relatively constant height above the substrate as it dispenses the no-flow flux and underfill. It is critical to the proper placement and function of the flip chip that no voids or air pockets be introduced between the flip chip and the substrate during the application process. Without the ability to produce a tailored topography of the no-flow flux and underfill material as it is applied to the substrate, undesirable voids or air pockets may be formed between the flip chip and the substrate.
A method is therefore needed for depositing no-flow flux and underfill material for flip chip attachment onto a substrate in which the dispensing apparatus does not physically contact the substrate. In addition, it would be advantageous for the method to allow tailoring the topography of the no-flow flux and underfill material as it is applied to the substrate. Such height tailoring will minimize voids and air pockets between the flip chip and the substrate.